1. Field of the Invention
The present invention relates to the formation of ultrafine particles of materials, and more particularly to the generation of micron and submicron particles by means of an electrohydrodynamic (EHD) process. The EHD process produces solid microspheres by solidification of molten droplets generated at a liquid surface stressed by intense electric fields.
2. Description of the Prior Art
At present there is no commercially available process for producing large quantities of micron and submicron powders from exotic alloys, high melting temperature materials such as Palladium, Tungsten or Molybdenum, and corrosive materials like uranium or titanium. Fine powder production using ultrasonic, centrifugal or gas atomization generally provides particulate material ranging from 10 to 100 microns in diameter. Although these methods are capable of producing commercial quantities of fine powders, they do not provide material with the ultrafine dimensions required of certain practical and research oriented applications. Other methods of producing fine powders are disclosed in U.S. Pat. Nos. 3,275,787 to Newberry, 3,975,184 to Akers, 3,963,812 to Schlienger, and 4,289,952 to Haggerty.
The uniqueness of the EHD process lies in its inherent capability for producing submicron spherical powders. Due to this feature, EHD technology has the potential of filling a significant void in the field of materials processing.
A prior art configuration employed to produce submicron particles by the EHD atomization process is illustrated in FIG. 1. Molten droplets are ejected from the tip of a nozzle 10 held at high potential. Small nozzle dimensions are required to achieve the high fields (10.sup.4 to 10.sup.5 V/cm) needed to overcome the surface tension forces holding the liquid surface together. Using nozzles with overall tip diameters of 0.025 cm, applied voltage of 10 kV are typically required to initiate and sustain the dispersion process. The nozzle is attached to a reservoir 12 containing a supply of feedstock material. The feedstock material is melted by means of a resistive heater 14 enclosing the reservoir, and molten material is fed to the nozzle apex region by application of positive pressure or by surface tension forces. A grounded extractor electrode 16 is positioned near the nozzle and the application of a high voltage to the nozzle creates a high field at the nozzle tip which pulls the liquid meniscus into a stable geometry. This geometry is indicated at 18 in FIG. 2 and is referred to as a Taylor cone. The applied electric field is further intensified at the apex of the liquid cone. The amplified electrostatic stresses soon exceed the surface tension forces, thereby causing the material to be dispersed into a divergent beam of positively charged submicron droplets. As is the case with any atomization process, droplets are ejected with a distribution in size. The conditions favorable for submicron particle generation are low material flowrates and wettability of the molten material with the nozzle emitter.
The process described above is disclosed in U.S. Pat. No. 4,264,641 to Mahoney et al., the disclosure of which is incorporated herein by reference. This process, is well suited for producing samples used in scanning or transmission electron microscope analyses. However, it is limited to materials with low or intermediate melting points (typically less than 2000.degree. C.) and production of powders in low quantities. Factors which limit the type of materials that can be reduced to submicron powder include wettability of molten material with source components, temperature limitations, crucible/nozzle corrosion and the inadequacies of resistive heating elements. Induction heating can be used to achieve high temperatures with a single source but it is difficult to integrate this heating method into a system using multiple powder generators. Furthermore, the power required by either resistive or inductive heating to maintain reservoir material in the molten state is excessive.
The present invention in one embodiment employs electron beam bombardment to heat a material which is to be atomized. Heating of materials in this fashion has been accomplished in the vacuum evaporation of metal films and films for high resolution shadowing of electron microscope specimens. These applications are discussed in J. Burden et al., "The Evaporation of Metals and Elemental Semiconductors Using a Work-Accelerated Electron Beam Source," Vacuum, 19 (1969) 397, and in Zingsheim et al., "Apparatus for Ultra-Shadowing of Freeze Etched Electron Microscope Specimens," J. Phys. E: Sci. Instrum., 3 (1970) 39. Electron bombardment heating has been used to heat an emitted tip of an EHD ion source, as described in T. Noda et al., "An Electrohydrodynamic Ion Source with a Reservoir and an Emitter Tip Heated by Electron Bombardment," Int. J. Mass Spectrometry and Ion Phys. 46 (1983) 15. This heating method has also been employed for nozzle heating in the prior art EHD process for forming submicron particles described above.